1. Field of the Invention
The present invention relates to liquid polyisocyanate mixtures based on 4,4'-diphenylmethane diisocyanate and containing allophanate groups, a process for their production and their use for the production of two-component polyurethane coating compositions.
2. Description of the Prior Art
Two-component polyurethane coating compositions based on diphenyl-methane diisocyanate are known and described, e.g., in "Wagner-Sarx, Lackkunstharze, 5th edition, Carl Hanser Verlag, Munich,
For the production of solvent-free two-component polyurethane systems which enable thick coatings to be produced in one operation, liquid polyisocyanate mixtures of the diphenylmethane series are generally used as the polyisocyanate component. These are usually undistilled and therefore strongly colored polyisocyanate mixtures that are obtained in known manner by the phosgenation of aniline/formaldehyde condensates.
For decorative applications, such as floor coatings, it is desirable to use light-colored diphenylmethane diisocyanate mixtures. Such liquid, light-colored polyisocyanates include mixtures of 4,4'-diphenylmethane diisocyanate with greater quantities of 2,4'-diphenylmethane diisocyanate. Mixtures of this type display poor resistance to crystallization at low temperatures, and also the 2,4'-diphenylmethane diisocyanate portion markedly impairs the properties of the coating materials produced therefrom. There is a demand for light-colored, non-crystallizing polyisocyanate mixtures which are based exclusively on 4,4'-diphenylmethane diisocyanate and are suitable for the purpose mentioned.
The production of modified polyisocyanate mixtures based on 4,4'-diphenyl-methane diisocyanate which are liquid at ambient temperature is known. For the production of mixtures of this type, according to DE-PS 1,618,380 for example, 1 mole 4,4'-diphenylmethane diisocyanate is reacted with 0.1 to 0.3 mole tri-1,2-oxypropylene glycol and/or poly-1,2-oxypropylene glycol having a molecular weight of up to 700. According to GB-PS 1,369,334 the modification is performed in two reaction steps and dipropylene glycol or polyoxypropylene glycol having a molecular weight of less than 2000 is used as the modifier.
DE-OS 2,913,126 describes diphenylmethane diisocyanate compositions in which 10 to 35% by weight of the isocyanate groups are reacted with a mixture of at least 3 alkylene glycols, wherein one of these glycols is di-, tri-or polypropylene glycol. In DE-OS 2,404,166 mixtures of polyoxyethylene glycol or polyoxypropylene glycol having an average molecular weight of less than 650 and at least one alkylene glycol having at least 3 C atoms are mentioned as modifiers. DE-OS 2.346,996 relates to diphenylmethane diisocyanate compositions in which 10 to 35 % by weight of the isocyanate groups are reacted with a commercial polyethylene glycol. Finally, according to EP-A 0,341,515, 4,4'-diphenyl-methane diisocyanate is modified with mixtures of aminopolyethers and tripropylene glycol.
All of the polyisocyanate mixtures described in these publications contain incorporated ether structures due to the incorporation of polyether polyols. From an application standpoint this is disadvantageous. Even though the incorporation of these polyols leads to an elasticizing of the finished products which is often undesirable, such polyoxyalkylene oxide units are known to be sensitive to oxidative degradation.
Liquid polyisocyanate mixtures based on 4,4'-diphenylmethane diisocyanate which do not contain any ether structures can be obtained by proportional carbodiimidization of 4,4'-diphenylmethane diisocyanate, as is described for example in DE-PS 1,092,007, DE-OS 2,537,685, U.S. Pat. No. 4,014,935, U.S. Pat. No. 4,088,665 and U.S. Pat. No. 4,154,752. This method also has disadvantages since even though product mixtures which have only a small proportion of carbodiimide structures display a sufficiently low viscosity, they suffer from poor resistance to crystallization, particularly at temperatures below 10.degree. C. Product mixtures which have a larger proportion of carbodiimide structures display satisfactory resistance to crystallization, but have to high a viscosity for many applications (cf. e.g. DE-OS 2,537,685, Examples 1 to 3).
In order to overcome these problems modification with polyalkylene oxide polyols is often undertaken in addition to the carbodiimide modification. However, this results in the disadvantages described.
It is an object of the present invention to provide low-viscosity polyisocyanate mixtures based on 4,4'-diphenylmethane diisocyanate which are non-crystallizing at 5.degree. C. and which do not contain any ether or polyalkylene oxide structures.
This object was achieved with the polyisocyanate mixtures containing allophanate groups described in more detail below.